The telephone subscriber loop is the portion of a telecommunications circuit which connects the central office to the customer premise equipment. The subscriber loop, often referred to as the local loop or "last mile", comprises a two-wire twisted-pair cable to provide full duplex communications between the subscriber and the central office.
The local loop is the most expensive, and perhaps the least technically effective, portion of the entire telecommunications system today. The subscriber loop is the most susceptible to transmission impairment. This is largely due to the subscriber loop being exposed to the elements as it hangs on poles to reach the end user. The local loop cable may also be buried in the ground in conduit, which is susceptible to being inadvertently cut or damaged. In such cases, the current-carrying portion of the cable may become wet, causing communications to degrade or cease to exist altogether.
The principal types of faults which can occur on the subscriber loop are short-circuits, power cross faults, grounds, and open-circuits. For example, a short-circuit would include the tip and ring lines being short-circuited together. A power cross involves a short-circuit between the tip and ring lines of two different telephone lines, or may involve a short-circuit between the telephone line and an electrical power line. A short-circuit between the ring line and ground is considered a ground fault, while a discontinuity in a line is referred to as an open cable pair. Power crosses and grounds are often the result of wet cable, which causes current flow between conductors.
There have traditionally been three categories of telephone subscriber line testing. First, local test desks (LTD) have been used to access a cable pair through the switching system at the central office. The LTD is a manually operated system which accesses the cable pair through a circuit in the switching system that connects the LTD to the line without ringing the telephone. Such a test requires dedicated test circuitry and a test board operator at the central office.
Another type of telephone subscriber line testing is automatic test equipment that accesses local loops through the switching system, similar to that of the LTD tests. Such a test is the line insulation test (LIT), which is performed at the central office by stepping through each telephone line and applying a line test.
Manual loop tests can be performed using a device called a time domain reflectometer (TDR). The TDR locates loop faults by sending a pulse on the telephone line, and displays the reflected pulse which can provide the distance to the fault. TDRs are generally very expensive.
In order to more closely locate the location at which a fault occurs on the telephone line, one must properly distinguish between outside plant troubles and problems with the subscriber's wiring and customer premises equipment (CPE). Network interface devices (NID) have been used at the interface between the telephone subscriber's wiring and the telephone company's equipment. The NIDs help to isolate the problem on the telephone line to the subscriber's wiring. One such subscriber line interface circuit is the advanced subscriber line interface circuit (ASLIC) Am79212/Am79C202 commercially available by Advanced Micro Devices, the assignee of the instant application.
When the telephone connection is in ring mode, the central office provides an indication that an incoming call is being directed to the subscriber. When the subscriber picks up the telephone, the connection is considered "off-hook", and a current is allowed to flow through the local loop. This current is detected, which initiates a "ring trip". A ring trip indicates that the ring signal is discontinued, traditionally by tripping the ring voltage relay. A current flowing during ringing could be a power cross fault, or could be an indication of a normal off-hook condition, either of which should cause a ring trip.
The aforementioned prior art does not provide for flexibility in performing AC and DC fault detection while in ring mode. The prior art requires complex analog and digital circuitry, thereby requiring additional, valuable semiconductor real estate. For example, the prior art does not lend itself to quick and inexpensive modification of fault thresholds.
The present invention provides a manner in which telephone power line faults which occur in ring mode may be monitored at the subscriber interface, while providing flexibility and ease of use. The present invention, therefore, provides a solution to the aforementioned and other shortcomings of the prior art, and provides additional advantages over the prior art.